As a process for forming a coating film for an automobile body, a process where an electrodeposited film is formed on an object to be coated and a coating film is then formed in accordance with three-coat and two-bake (3C2B) process including coating of an intermediate coating composition→baking and curing→coating of a base coating composition→coating of a clear coating composition→baking and curing has been widely adopted. Recently, in terms of energy-saving, etc., a process where the step of baking and curing after coating an intermediate coating composition is omitted, and an electrodeposited film is formed on an object to be coated and a multi-layered coating film is then formed in accordance with three-coat and one-bake (3C1B) process including coating of an intermediate coating composition→coating of a base coating composition→coating of a clear coating composition→baking and curing has been attempted (for example, Patent Document 1).
However, in the aforementioned 3C1B process, since a mixed layer of the intermediate coating film and a base coating film is apt to be generated, the smoothness and the chipping resistance of the obtained coating film are lowered, causing a problem.
As its countermeasure, for example, in Patent Document 1, it is described that in the aforementioned 3C1B process, when a coating composition containing (A) a vinylic copolymer that includes a specific amount of a specific structure unit, has a hydroxyl group value of 60-200 mgKOH/g and has a number-average molecular weight of 2,000-6,000, (B) a polyester resin that has a hydroxyl group value of 80-120 mgKOH/g and a number-average molecular weight of 1,500-2,600, (C) a nonaqueous polymer dispersion, and (D) one or more crosslinkable resins selected from the group consisting of an alkyl etherified melamine resin and blocked polyisocyanate resin is used as a thermosetting intermediate coating composition, film performances such as appearance, chipping resistance, adhesion, and weather resistance are not lowered.
In addition, in Patent Document 2, it is described that in the aforementioned 3C1B process, when a coating composition containing (a) 40-56 wt % of a urethane-modified polyester resin with a number-average molecular weight of 1,500-3,000, that is obtained by reacting a polyester resin containing a hydroxyl group, which is obtained by the polycondensation of an acid component containing 80 mol % or more of isophthalic acid and a polyhydric alcohol and has a glass transition point (Tg) of 40-80° C., and an aliphatic diisocyanate compound, (b) 10-30 wt % of a melamine resin, (c) 15-30 wt % of an isocyanate compound in which hexamethylene diisocyanate and an isocyanate compound, which is obtained by reacting hexamethylene diisocyanate with a compound reactive thereto, are blocked by a compound having an active methylene group, (d) 4-15 wt % of a nonaqueous dispersion resin having a core-shell structure (the amounts of (a)-(d) are based on the solid content weight of the coating resin), and (e) 0.4-2 parts by weight of a flat pigment with a major diameter of 1-10 μm and a number-average particle diameter of 2-6 μm (the solid content weight of the coating resin is assumed as 100 parts by weight) is used as an intermediate coating composition, a multi-layered coating film having an excellent finish appearance and chipping resistance can be formed.